Conventionally, wrap films made of a polyvinylidene chloride-based resin have been used as materials for the simple packaging of foods and the like in many ordinary households because the films have excellent adhesion to adherends and each other and properties such as gas barrier properties.
Usually, an inflation film formation process is widely used to produce a wrap film made of a polyvinylidene chloride-based resin. In an inflation film formation process, the resin is extruded from a die in tubular form, and then the outer side of the resin in tubular form is usually brought into contact with a coolant such as cold water filling a storage tank called a cold water bath. At the time, the coolant is injected into and retained in the inside of the tubular (cylindrical) resin held between a die opening and a pinch roll, and at the same time the inner side of the resin is brought into contact with a coolant such as a mineral oil to cool and solidify the resin into a film. As used herein, the tubular resin portion held between the die opening and the pinch roll is called a sock. The coolant (liquid) injected into the inside of the sock is called a sock liquid. Here, the sock is collapsed by the pinch roll to form a tubular double-ply sheet, and this double-ply sheet is called a parison.
To obtain a stretched film by the production process above, this parison is reheated and stretched by blowing air into the inside of the parison (inflation) (hereinafter also referred to as inflation stretching). At the time, reopening the parison collapsed once requires providing the sock liquid with the lubricating effect as an opening agent. In addition, separating the stretched double-ply film into a single-ply film also requires providing the sock liquid with the lubricating effect as an opening agent.
In general, for inflation stretching, a method that uses a mineral oil as the sock liquid is known. Because a mineral oil has a lower specific gravity than water, cooling increases the pressure from the outer side to the inner side of the sock, making it impossible to provide the sock with tension. As a result, unfortunately, when the sock is collapsed by the pinch roll to form a parison, wrinkles (called parison wrinkles) form thereon, inducing a blowout due to inflation stretching, and making the sock unstable because the water pressure of the cold water bath causes the sock to pulsate and the like.
To solve these problems, a method that uses a mineral oil along with water, ethylene glycol, or propylene glycol is widely known, as disclosed by Patent Document 1.
Moreover, Patent Document 2 proposes a technique using a cellulose-based aqueous solution and Patent Document 3 proposes a technique using a polyvinyl alcohol-based aqueous solution as a sock liquid that uses no mineral oil having a low specific gravity, has the lubricating effect as an opening agent, and forms one-component.
In addition, Patent Document 4 describes a conventional technique using propylene glycol, glycerin, or the like in undiluted form or in aqueous form as a sock liquid.
Moreover, Patent Document 5 describes that propylene glycol or dipropylene glycol is used as a solvent for a dusting agent. Patent Document 5 describes that the use of an inorganic powder for a dusting agent improves the opening properties of a parison.    Patent Document 1: Japanese Patent Publication No. 27-2793    Patent Document 2: Published Japanese Translation No. 2002-539986 of the PCT International Application    Patent Document 3: Japanese Patent Application Laid-Open No. 2002-331623    Patent Document 4: Japanese Patent Application Laid-Open No. 10-330625    Patent Document 5: Japanese Patent Application Laid-Open No. 2-49036